Multiple programmable current sources are generally fabricated as an integrated circuit chip designed to accept data and control signals as inputs and deliver constant, electric currents as outputs to other electronic circuits, such as light-emissive displays and other systems requiring multiple, programmable current sources as inputs. Using light-emissive displays as an example, such displays generally consist of a matrix of rows and columns of pixels (from hundreds to thousands, depending upon the size and definition of the display). The rows are generally designated 0 through R and the columns are designated 0 through N. It should be understood that rows and columns can be interchanged throughout this disclosure and no use of the terms ‘rows’ or ‘columns’ is intended in any way to limit the structure or the scope of the invention.
A complete screen is usually produced by cycling the rows periodically from 0 to R and supplying to each column an amount of current required to produce the desired luminance in each pixel in the activated row. Thus, for example, when row 0 is activated a current is supplied to each column 0 through N, when row 1 is activated a different current (possibly) is supplied to each column 0 through N, etc. Since only one row of pixels is activated at a time, only the pixels in that row are driven to emit light by the current applied to the columns. The current applied to each column generally has a plurality of steps or levels that cause the light emissive device in a pixel to emit light in an equal number of steps or levels. During the process of addressing each pixel in a screen, it is desirable for each pixel to have the same brightness or luminance for each step or level.
In many prior art constant current sources a skew or mismatch is exhibited due to intrinsic device parameters in the constant current source circuitry. These skews or mismatches are undesirable and can occur for any type of transistor topology, including but not limited to silicon based NMOS or PMOS transistors. Also, while many different implementations of light-emissive displays (and other systems requiring multiple, programmable current sources) are possible, in most instances the current must flow through a plurality of components (e.g. transistors, capacitances, etc.) connected in series. The intrinsic mismatch between these series components and subsequent series components (e.g. between pixels in a column) causes current variations between the currents flowing in different columns, which produce undesired variations in luminance of the pixels.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved programmable current source.
Another object of the invention is to provide a new and improved programmable current source that provides matched currents at multiple outputs.
And another object of the invention is to provide a new and improved programmable current source and a variety of calibration techniques that provides matched currents at multiple outputs.
Still another object of the present invention is to provide a new and improved programmable current source that can be easily incorporated with virtually any circuitry requiring multiple constant current outputs.
Yet another object of the invention is to provide a new and improved programmable current source that is relatively easy and inexpensive to integrate on a single chip.